Spatial pattern in the precipitation regimes of Nepal

A large-scale perspective is provided upon the nature of precipitation regimes across Nepal by classifying long-term mean monthly precipitation data for 222 stations. The classification methodology is shown to be a useful tool for identifying underlying spatial structure in precipitation regime shape (seasonal variability) and magnitude (size) in an extreme physical environment where climatological patterns are complex and poorly known. Four precipitation regime shape classes are identified: (a) July-August peaks are typical of western regions; (b) marked July peaks are characteristic of the central region and eastern lowlands; (c) July peaks with gradual onset are mainly concentrated in eastern Middle Mountains; and (d) July-August peaks with winter precipitation are confined to western higher mountains, particularly the Trans-Himalayan region. Four precipitation regime magnitude classes are found: (a) low regimes clearly dominate western high-mountain areas but occur more widely across the High Mountains and High Himalaya; (b) intermediate regimes are distributed from east to west at low attitude, dominating the western Terai/Siwaliks; (c) moderately high regimes are widespread but most evident in the Terai (plains) to Middle Mountains of central and eastern parts and eastern high mountains; and (d) high precipitation zones are situated near Pokhara and northeast of the Kathmandu Valley around Langtang. A composite (shape and magnitude) regime classification indicates the key controls upon spatial patterns in Nepalese precipitation to be: length and timing of the summer monsoon (duration decreases east to west with later onset and earlier withdrawal in the west); successively higher altitude, east-west-trending mountain ranges causing rainfall to decline broadly south-north; topographic barriers that induce local rain shadows (lee) and precipitation hotspots (windward side); and westerly weather systems supplying winter precipitation to the northwest mountains. Although general patterns relating to zonal movement of the summer monsoon and physiographic (mountain ranges) controls upon precipitation may be identified, the role of mountainous relief in yielding localized precipitation patterns is significant. Copyright (C) 2004 Royal Meteorological Society.